The invention relates to the manufacture of rotary drill bits for use in drilling or coring holes in subsurface formations.
The invention may be applied to any rotary drill bit of the kind having cutting inserts mounted in sockets in a part of the bit body. However, the invention is particularly applicable to drag-type rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutting structures mounted in sockets in the bit body and projecting from the face of the bit, and a number of nozzles, also mounted in sockets in the bit body, and communicating with a passage for supplying fluid to the face of the bit.
Each cutting structure may comprise a cutting element mounted on a carrier, such as a stud or post, which is received in the socket in the bit body. One common form of cutting element comprises a circular tablet having a facing table of polycrystalline diamond or other superhard material and a substrate of less hard material such as cemented tungsten carbide.
Particularly in cases where the bit body is machined from steel, the surface of such a bit is susceptible to wear and erosion during use, particularly in the vicinity of the nozzles from which abrasive drilling fluid emerges at high velocity and with substantial turbulence. Accordingly, it is fairly common practice to apply a hard facing material to the surface of the bit body, at least around the cutting structures. Normally the hard facing is applied to the bit body before the cutting structures themselves are fitted into their sockets since the cutting structures would be likely to suffer thermal damage if the hard facing were to be applied after the cutting structures had been fitted.
In order to prevent the hard facing material, which is applied in a molten state, from entering the sockets, it is usual to plug the sockets temporarily with formers made of carbon material, such as graphite. The hard facing material is then applied to the surface of the bit body around the formers, usually by a welding process using a gas welding arc or oxyacetylene torch.
However, in the methods normally employed, the molten hard facing material, during the welding process, does not wet the surfaces of the carbon formers with the result that a meniscus is formed between each former and the solidified hard facing material, resulting in the formation of a depression in the hard facing material around the former, after the material has solidified. The existence of this depression may facilitate removal of the formers after the hard facing has solidified, but it means that there is then a similar depression in the hard facing material around the subsequently inserted cutting structure. The presence of this depression enhances the erosive effect of the drilling fluid flowing around the cutting structures with the result that, in practice, there is accelerated and preferential erosion of the cutting structure and hard facing material, leading to premature failure.
The present invention provides an improved method of applying hard facing material to a bit body which reduces or eliminates the formation of a depression in the hard facing around each former, and hence between the cutting structures and the hard facing, and thus reduces or eliminates the preferential erosion which can occur with the prior art methods.